1. Field of the Invention
The present invention relates to an image forming method for use in producing toner images in image forming apparatus such as printers, copiers and facsimile machines. More particularly, the present invention relates to an image forming method using an electromagnetic induction toner fixing method.
2. Discussion of the Background
Image forming apparatus, which produce toner images using an image forming method such as electrophotography, electrostatic recording and magnetic recording, have been widely used as printers, copiers and facsimile machines. The image forming apparatus typically perform the following image forming operations:    (1) a toner image is formed on a receiving material such as plain papers, photosensitive materials and electrostatic recording materials, using a direct or indirect image transfer method; and    (2) the toner image is fixed on the receiving material by a heat fixing method such as heat roller fixing methods, film fixing methods and electromagnetic induction toner fixing method.
Recently, there are various needs (such as energy saving and high speed image formation) for the image forming apparatus. In order to fulfill the needs, it is important to improve the heat efficiency of the fixing device used for the image forming apparatus.
Heat roller fixing devices typically include a fixing roller which includes a heat source such as halogen lamps therein and which is heated by the heat source while the temperature thereof is controlled so as to be a predetermined temperature, and a pressure roller which is rotated while contacting the heat roller. A receiving material bearing an unfixed toner image thereon is fed through the nip between the pair of heat roller and pressure roller so that the toner image is melted and fixed thereon by the heat and pressure applied by the pair of rollers.
Heat fixing devices using a film have been disclosed in published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 63-313182 and 01-263679. In such heat fixing devices, a receiving material bearing a toner image thereon is heated by being contacted with a thin heat resistant film which is rotated while contacted with a heating member supported by a support, so that the toner image is fixed on the receiving material. Suitable heaters for use as the heating member include ceramic heaters having a constitution such that a resistive element is provided on a ceramic substrate such as alumina and aluminum nitride which has good heat resistance, good insulating property and good heat conductivity. Since the film is thin and has low heat capacity, the film heat fixing devices have advantages over the heat roller fixing devices such that the fixing devices have a relatively high heat transfer efficiency and short warm-up time and thereby image forming operations can be quickly started, resulting in energy saving.
JP-A 08-22206 discloses an electromagnetic induction heat fixing technique in that an alternate magnetic field is applied to a heating member including a magnetic metallic member to generate eddy current therein, resulting in generation of Joule heat therein.
A conventional electromagnetic induction heat fixing device will be explained referring to a drawing.
FIG. 3 is a schematic view illustrating a conventional electromagnetic induction heat fixing device. The heat fixing device includes a film guide 21 containing a heating member 20 having an excitation coil unit 18 and a magnetic metallic member 19 serving as a heater; a heat resistant cylindrical film 17 which rotates while the inner surface thereof is contacted with the outer surface of the film guide 21 at a position of the magnetic metallic member 19; and a pressure roller 22 which is contacted with the film 17 while forming a nip N therebetween and which rotates the film 17.
Suitable films for use as the film 17 include heat resistant films having a thickness not greater than 100 μm, and preferably from 20 to 50 μm. Specific examples thereof include films made of a polytetrafluoroethylene (PTFE), a perfluoroethylene/perfluoroalkoxyethylene copolymer (PFA), or a fluoroethylene/propylene copolymer (FEP); and complex films in which a resin such as PTFE, PFA or FEP is coated on a peripheral surface of a film such as polyimides, polyamideimide, polyether ether ketone (PEEK), polyether sulfone (PES) and polyphenylene sulfide (PPS).
The film guide 21 is formed of a resin material having a high heat resistance and a high stiffness, such as PEEK and PPS. The heating member 20 is set in substantially the center portion of the film guide 20 and extends in the longitudinal direction of the film guide 20.
The pressure roller 22 includes a core 22a and a heat resistant rubber layer 22b which is made of a rubber with good releasability such as silicone rubbers and which is formed on the core 22a. The pressure roller 22 is pressed by a pressing member or a bearing (both are not shown) such that the film 17 is pressed to the magnetic metallic member 19 of the heating member 20. The pressure roller 22 is rotated counterclockwise by a driving device (not shown).
When the pressure roller 22 is rotated, the film 17 is also rotated due to friction between the film 17 and the pressure roller 22. Thus, the film 17 is rotated while contacting the heating member 20.
When the heating member 20 is heated to a predetermined temperature, a receiving material 11 bearing an unfixed toner image T thereon which is formed by an image forming section (not shown) is fed into the nip N between the film 17 and the pressure roller 22. The heat generated by the magnetic metallic member 19 of the heating member 20 is applied to the receiving material 11 via the film 17, and thereby the toner image T is melted and fixed on the receiving material 11. Then the receiving material 11 is separated from the surface of the film 17 at the exit of the nip N, and is fed to a discharge tray (not shown).
In the electromagnetic induction heat fixing device, the magnetic metallic member 19 can be set at a position closer to the toner image on the receiving material than in the case of the above-mentioned heating device using a film because heat generated by eddy current is used for the electromagnetic induction heat fixing device. Therefore, the electromagnetic induction heat fixing device has better heat efficiency than the heating device using a film.
When full color images are fixed by a fixing device, the fixing device has to have an ability to sufficiently melt four or more color toner layers, which are overlaid on a receiving material, to fix the color toner layers on the receiving material. In order to fulfill the requirement, the electromagnetic induction heat fixing device typically uses a film having an elastic layer with a certain thickness thereon, to heat and melt the toner layers while enveloping the toner layers. When a silicone rubber is formed on the surface of the film as the elastic layer, the heat response of the film deteriorates because the elastic layer has poor heat conductivity. When a large amount of toner images are fixed by such a fixing device, the temperature of the outer surface of the film rapidly falls, and thereby a fixing problem in that the toner images are insufficiently fixed and/or a cold offset problem in that the toner images adhere to the film and then re-transferred to an undesired position of the receiving sheet or another receiving sheet occur.
In image forming methods using a toner, electrostatic or magnetic latent images are developed with the toner. For example, in electrophotography, an electrostatic latent image, which is formed on a photoreceptor, is developed with a toner, resulting in formation of a toner image on the photoreceptor. The toner image is typically transferred to a receiving material such as papers, and then fixed thereon upon application of heat thereto.
The toner used for developing electrostatic latent images typically includes colored particles in which a colorant, a charge controlling agent and additives are dispersed in a binder resin. The methods for producing toners are broadly classified into pulverization methods and suspension polymerization methods.
Pulverization methods typically include the following processes:    (1) toner constituents such as a thermoplastic resin (serving as a binder resin), a colorant, a charge controlling agent and additives such as an offset preventing agent are heated and kneaded to disperse the colorant, charge controlling agent and additives in the thermoplastic resin;    (2) the kneaded mixture is cooled and then pulverized;    (3) the pulverized mixture is classified, resulting in preparation of toner particles.
The toners prepared by the pulverization methods have fairly good characteristics, but only limited materials can be used as the toner constituents. Specifically, the kneaded mixture has to be easily pulverized by a general economic pulverizer and the resultant powder has to be classified by a general economic classifier. Namely, the kneaded mixture has to be brittle. Therefore, when the kneaded mixture is pulverized, the resultant powder has a wide particle diameter distribution. In order to produce toner images having good resolution, the toner particles preferably have a particle diameter of from about 5 μm to about 20 μm, and therefore fine particles having a particle diameter, for example, less than 5 μm and coarse particles having a particle diameter, for example, greater than 20 μm are preferably removed. Therefore, the yield of the toner is low. In addition, when the pulverization methods are used for producing a toner, it is difficult to uniformly disperse materials such as colorants and charge controlling agents in a binder resin. In this case, the resultant toner has poor fluidity, developability and durability and therefore the resultant toner images have poor image qualities.
In attempting to solve such problems, toner manufacturing methods using suspension polymerization techniques have been proposed and are practically used now. However, the resultant toners typically have a spherical form, and have poor cleanability. When images with a low image area proportion are developed using such toners, no problem occurs. However, when images (such as pictorial images) with a high image area proportion are developed, or when a receiving paper is jammed while the toner image is not transferred to a receiving paper, a large amount of toner particles remain on the photoreceptor used. Since the residual toner particles cannot be well removed from the surface of the photoreceptor, and thereby the resultant toner images have undesired background development. In addition, since the contact area of toner particles decreases due to spherical form of the toner particles, and thereby the adhesion of the toner particles to receiving materials decreases, resulting in deterioration of fixing flexibility of the toner. Specifically, the resultant toner has unstable low temperature fixability.
In attempting to solve the problem, a method in which a particulate resin prepared by an emulsion polymerization method is aggregated to prepare toner particles having irregular forms is proposed in Japanese Patent No. 2,537,503 (i.e., JP-A 63-186253). However, the thus prepared toner particles include a large amount of surfactant on the surface thereof and therein, and therefore the toner has poor charge stability when environmental conditions (such as humidity) change. In addition, the toner has a wide charge quantity distribution, and thereby the background development problem in which background of images is soiled with toner particles is caused. Further, the image forming devices such as photoreceptors, chargers, and developing rollers are contaminated by the surfactant included in the toner, and thereby problems such that the abilities of the devices cannot be well exhibited occur.
In the fixing process, toner is required to have a good releasability (hereinafter referred to as offset resistance) against the heating members. In order to enhance the releasability of a toner, it is important that a release agent is present on the surface of the toner. Specifically, JP-As 2000-292973 and 2000-292978 have disclosed a technique in that a particulate resin is included in the toner particles such that the concentration of the particulate resin is higher in the surface portion of the toner particles than that in the other portions of the toner particles. However, the lowest fixable temperature of the toner is relatively high, namely the toner has poor low temperature fixability and poor energy saving property.
The toner prepared by the method, in which a particulate resin prepared by an emulsion polymerization method is aggregated to prepare toner particles having irregular form, has the following drawbacks:    (1) when the particulate resin is aggregated together with a particulate releasing agent and a colorant, the releasing agent tends to be included in the particulate resin, and thereby good releasability cannot be imparted to the resultant toner;    (2) since toner particles are prepared while the particles of the particulate resin, particulate releasing agent and colorant are randomly fused to the other particles, the formula of the resultant toner particles and the molecular weight of the binder resin included in the toner particles vary, and thereby the surface properties of the toner particles vary. Therefore, it is impossible for the toner to produce high quality images for a long period of time. In addition, when the toner is used for low temperature fixing devices, the cold offset problem tends to occur because the particulate resin present on the surface of the toner particles deteriorates the low temperature fixability of the toner.
Further, when the toner is used for fixing devices using a film on which an elastic layer having low heat conductivity is formed, a smear problem in that when the fixed toner image is rubbed, the toner image is damaged (the toner image is broken at the intermediate portion thereof) and thereby the background area is smeared is caused. This is because the fixing device has poor heat response as mentioned above and therefore the toner particles in the toner image are insufficiently fused with each other although the toner particles in the upper portion of the toner image are fused to the adjacent toner particles and the adhesion of the toner particles in the bottom portion of the toner image to the receiving material is acceptable due to heat and pressure of the pressure roller.
Because of these reasons, a need exists for an image forming method by which high quality images can be produced for a long period of time without causing the offset problem, low temperature fixing problem and smear problem.